[franken]

Doesn't turn over is unclear.
If the solenoid clicks, or nothing happens, it could be a solenoid problem.
If the starter turns slowly, its probably not the solenoid.

Let's think about what's happening...
The ignition switch turns on the solenoid, which (is an electric switch that) turns on the starter motor.

If nothing happens or the solenoid clicks, and that's all, it may be a problem with getting enough current from the battery all the way to the solenoid.

Check the big ground cable by jumping from the neg batt post to the starter case w/ one side of jumper cables and test by starting or trying to. We like the ground to go to the engine rather than something bolted to it. A good spot is the holes at the front of the engine below the water pump that were once for motor mounts.

Another test is to feel the cable ends for heat after cranking for a bit when cold. Resistive connections will get hot.

Then there are all the positive side connections from the smaller wire coming off the battery all the way to when it goes through the ignition switch (turns purple) NSS and on to the solenoid. All those connections should be clean, and not get hot after cranking when cold.

A Ford solenoid won't help a slow starter, unless its a wiring issue that was fixed accidentally by wiring the solenoid in, or if the lugs and disk in the solenoid are burned. In that case the extra solenoid was a waste.

The point is, a Ford solenoid is an added complexity that will usually not be needed if the problem is approached methodically. Its also something that has to be troubleshot next time there's a cranking problem.

Troubleshoot or throw parts at the problem...